1. Field of the Invention
The present invention relates to a hold-down spring unit for top nozzles of nuclear fuel assemblies which are used in nuclear reactors. The hold-down spring unit has an improved hold-down performance to prevent the nuclear fuel assembly from lifting up. The present invention, also, relates to a top nozzle for nuclear fuel assemblies which employ the hold-down spring unit.
2. Description of the Related Art
As is well known to those skilled in the art, a nuclear reactor is a device in which a fission chain reaction of fissionable materials is controlled for the purpose of generating heat, producing radioactive isotopes and plutonium, or forming a radiation field.
Generally, in light-water reactor nuclear power plants, enriched uranium which is increased in the ratio of uranium-235 to 2% through 5%, is used. To process enriched uranium into nuclear fuel to be used in nuclear reactors, a forming process is conducted by which uranium is formed into a cylindrical pellet having a weight of about 5 g. Several hundreds of these pellets are retained into a bundle and inserted into a zirconium tube under vacuum conditions. A spring and a helium gas are supplied into the tube and a cover is welded and sealed onto the tube, thus completing a fuel rod. A plurality of fuel rods constitutes a nuclear fuel assembly and is burned in a nuclear reactor by nuclear reaction.
FIG. 1 is a front view showing a conventional nuclear fuel assembly.
As shown in FIG. 1, the nuclear fuel assembly includes a plurality of support grids 10 through which fuel rods (not shown) are inserted, and a plurality of guide thimbles 15, which are coupled to the support grids 10. The nuclear fuel assembly further includes a top nozzle 30, which is coupled to the upper ends of the guide thimbles 15, a bottom nozzle 16, which is coupled to the lower ends of the guide thimbles 15, and the fuel rods (not shown), which are supported by springs and dimples that are formed in the support grids 10.
To assemble the nuclear fuel assembly having the above-mentioned construction, lacquer is applied to the surfaces of the fuel rods to prevent the fuel rods from being scratched and to prevent springs provided in the support grids 10 from being damaged. Thereafter, the fuel rods are inserted through the support grids 10 and then the top nozzles 30 and bottom nozzles 16 are coupled to the fuel rods, thus completing the assembly of the nuclear fuel assembly. The assembled nuclear fuel assembly is tested for distances between the fuel rods, distortion, dimensions including the length, etc. after the lacquer is removed. When the results of the test are normal, the nuclear fuel assembly is installed in a core of a nuclear reactor in which nuclear fission is produced, as disclosed in U.S. Pat. No. 5,213,757.
In the nuclear fuel assembly installed in the core, a hydraulic uplift force which is generated by the flow of coolant during the operation of the nuclear reactor is applied to the top nozzles 30 and bottom nozzles 16. Hereby, the nuclear fuel assembly is lifted up or vibrated. Furthermore, thermal expansion attributable to an increase in temperature, irradiation growth of the nuclear fuel tube as a result of neutron irradiation for a long period of time, or axial length variation caused by creep may be induced. Therefore, the top nozzle 30 is configured to ensure the mechanical and structural stability of the nuclear fuel assembly with respect to axial movement or axial length variation of the nuclear fuel assembly.
FIG. 2 is a perspective view of the top nozzle 30 according to a conventional technique.
As shown in FIG. 2, the top nozzle 30 includes a plurality of spring clamps 31 which support hold-down spring units 32. Spring insert holes 31a are formed in each spring clamp 31. The ends of the hold-down spring units 32 are inserted into corresponding spring insert holes 31a. A fastening pin hole 32a″ is vertically formed through the end of each hold-down spring unit 32 which is inserted into the corresponding spring insert hole 31a. Each hold-down spring unit 32 includes a first spring 32a having a first neck part 32a′, a second spring 32b and a third spring 32c which are coupled to the first neck part 32a′. The hold-down spring unit 32 is configured such that the first, second and third springs 32a, 32b and 32c are stacked on top of one another. To couple the hold-down spring unit 32 to the top nozzle 30, a spring junction end of the hold-down spring unit 32 which is opposite the first neck part 32a′ is inserted into the corresponding spring insert hole 31a in the horizontal direction. Thereafter, a fastening pin 33 is inserted into the corresponding fastening pin hole 33′ of the spring clamp 31 and a fastening pin hole 32a″ of the hold-down spring unit 32 in the vertical direction. Thereby, the hold-down spring unit 32 is fastened to the top nozzle 30. Here, to prevent the fastening pin 33 from being removed, the fastening pin 33 is welded to an upper surface of the spring clamp 31.
As shown in FIG. 1, the top nozzle 30 having the above-mentioned construction is assembled with the elements of the nuclear fuel assembly. Subsequently, as is well known, the nuclear fuel assembly is installed in a core and disposed between an upper core plate (not shown) and a lower core plate such that the hold-down spring units 32 are supported by the lower surface of the upper core plate.
As shown in FIG. 2, the hold-down spring units 32 which are provided on the top nozzle 30 provide elastic force to the nuclear fuel assembly in response to axial movement or variation in the length of the nuclear fuel assembly so as to ensure the mechanical-structural stability of the nuclear fuel assembly. The first neck part 32a′ of the first spring 32a is inserted into an insert slot 41 formed in a corresponding upper plate 40 of the top nozzle 30 in order to guide the operation of the hold-down spring unit 32 and prevent a loss of an element when the first, second or third spring 32a, 32b or 32c is damaged.
FIG. 3 is a graph showing the characteristic curve of the hold-down spring unit 32 according to the conventional technique.
As shown in FIG. 3, the hold-down spring unit 32 according to the conventional technique has the hold-down margin such that the spring force is greater than the demand hold-down force in the entire operating section and the gradient of the graph showing the spring force as a function of displacement is constant. In other words, in the hold-down spring unit 32 mounted to the top nozzle 30 according to the conventional technique, because the first, second and third springs 32a, 32b and 32c apply resistance force to the nuclear fuel assembly at the same time, the hold-down margin is provided such that the gradient of the graph showing the spring force as a function of displacement is constant. Therefore, as shown in FIG. 3, to satisfy the hold-down margin under start-up conditions, the hold-down margin under hot full power conditions becomes excessively large. As a result of the excessive hold-down margin under hot full power conditions, the hold-down spring unit 32 applies an excessive resistance force to the nuclear fuel assembly, thus deteriorating the mechanical and structural stability of the nuclear fuel assembly.